Single strands can be made of metallic as well as nonmetallic materials, or hybrids thereof, such as insulated strands. Also, the so-called fillers (nonmetallic strands whose inherent function is for filling the rope or cable cross-section) are considered to be single strands as well.
Ropes or cables are manufactured on stranding machines on which are fitted bobbins carrying the single strands and in which these bobbins themselves are mounted on a rack, which is sometimes referred to as a carriage. The rope or cable is fabricated by rotating the carriage and simultaneously drawing out the strand. However, as is known, it is also possible to carry out stranding when the carriage is fixed by rotating the entire rope take-up device and drawing out the strand.
Independent of the type of stranding principle employed, an electromechanical pick-up normally has been provided for each strand used to form the rope or cable in order to monitor the individual strands. These pick-ups are designed to produce an electrical pulse on failure of the strand tension resulting from breakage or run-out of the strand, and this electrical pulse is used to stop the stranding process. Despite the advantages afforded by this electromechanical monitoring arrangement, such as simple construction, good operational reliability and independence of strand material, this type of monitoring also has various disadvantages. In this regard, for transmitting the pick-up signals from the rotating carriage to the monitoring and control equipment, wiper rings are required with their sufficiently well-known shortcomings. Also, in the event of a strand breaking, the relevant strand section in the vicinity of the pick-up can remain tensioned, so that the pick-up is not influenced by this strand section and the break in the strand goes undetected.
Other monitoring systems which have become known use a proximity detector operating on a capacitive, inductive or optical principle, located at a short distance in front of the stranding point. With each rotation of the carriage carrying the bobbins supplying the single strands, the correct number of single strands must be constantly sensed by the proximity detector, and, in the event of one or more missing strands, a machine stop signal is generated by the associated electronic system. This type of monitoring system requires only a small number of mechanical and electronic parts, and also has the disadvantage of dispensing with any need for the unreliable wiper rings required in other systems. On the other hand, strands breaking in the region of or after the stranding point and which stick at any part, go undetected. Furthermore, adjustment of the proximity detector or the data to be ascertained by it is necessary if the strand material is changed.